What Is eGFR in Blood Test Results? What It Measures, What’s Normal, and What It Means for Athletes
Table of Contents
Key Takeaways: What Is eGFR in Blood Test
- eGFR (estimated glomerular filtration rate) is a calculated estimate of how well your kidneys filter blood. In a routine blood test, eGFR is usually derived from serum creatinine together with age and sex.
- An eGFR of 90 or above sits in the “normal or high” category, 60–89 is “mildly decreased”, and a value below 60 that persists for at least three months meets the kidney-function threshold for chronic kidney disease (CKD).
- One low result is not a diagnosis. Guidelines advise repeating the tests to confirm whether kidney disease is present and assessing kidney function together with a urine albumin test.
- Creatinine-based eGFR can be misleading when muscle mass, diet or creatine supplements raise creatinine without necessarily reflecting a change in kidney filtration. Recent hard exercise can also raise creatinine temporarily, but a post-race rise is not always harmless.
- Athletes are a textbook example. When the number does not fit the person, cystatin C or the combined creatinine–cystatin C equation can give a clearer answer.
- A borderline eGFR should be neither ignored nor panicked over. Depending on the situation, sensible next steps may include repeat testing under standardized conditions, a urine albumin test, comparison with your own baseline and watching the trend.
- In my clinical experience, a flagged eGFR is common, particularly in men and in large-framed or muscular people, and the circumstances of the sample, such as fasting and drinking less than usual beforehand, may also play a part.
- Clearly or persistently abnormal results, or raised urine albumin, need proper evaluation. In my practice, this typically means a urine albumin-to-creatinine ratio, cystatin C, an assessment of possible causes such as diabetes and, when the findings warrant it, a nephrology referral.
- For most people, eGFR remains the practical basis for adjusting medication doses to kidney function, although in unusually large or very frail patients both body size and muscle mass may need to be considered, and some situations call for a more accurate estimate.
Introduction: What Is eGFR in Blood Test
Chronic kidney disease is far more common than most people realise. A Global Burden of Disease analysis estimated that in 2017 there were 697.5 million cases of all-stage CKD worldwide, a global prevalence of 9.1%.[1] Yet the KDIGO guideline points out that awareness remains very low: worldwide, only 6% of the general population and 10% of people in high-risk groups know their CKD status.[2]
For many people, the first contact with kidney function is a single line on a routine lab report. Many laboratories calculate and report eGFR automatically whenever serum creatinine is measured,[3] so it appears even when nobody specifically asked about the kidneys. That is exactly why the question “what is eGFR in a blood test, and should I worry about mine?” comes up so often.
As both a physician and a former competitive athlete, I see this question from two directions. For the general population, eGFR is a useful screening number on a basic panel. For muscular, highly trained or creatine-using people, the same number can look alarming for reasons that have little to do with the kidneys.
This article explains what eGFR in a blood test actually measures, how the result is categorised, why one low value is not a diagnosis, what can distort the estimate, and what all of this means for athletes.
In my clinical work, eGFR is a result I interpret almost every day, and almost every day it is outside the reference range for some patient. Because the value is reported automatically alongside creatinine, it often appears on a report even when nobody was specifically looking at the kidneys, and explaining why it has been flagged is a regular part of my consultations. In my experience, some patient groups receive a flagged eGFR noticeably more often than others.
What Is eGFR in a Blood Test? What the Number Actually Measures
The eGFR on a blood test report is an estimate of glomerular filtration rate (GFR). GFR describes how much blood plasma the kidneys’ filtering units clear per unit of time, and it is accepted as the best overall measure of kidney function.[3] Measuring it directly is cumbersome: it requires the urinary or plasma clearance of an ideal filtration marker such as inulin,[3] which is not practical for everyday blood work.
So in routine care, GFR is estimated. The 2024 KDIGO (Kidney Disease: Improving Global Outcomes) guideline advises using serum creatinine and an estimating equation for the initial assessment of GFR, and recommends relying on a validated equation rather than on the serum marker alone.[4] The result is reported in mL/min per 1.73 m² of body-surface area.[5]
The weak point is the input. Creatinine is directly linked to muscle mass,[2] but standard creatinine-based eGFR equations use age and sex without accounting for an individual’s actual muscle mass. The estimate can therefore be less accurate in people whose muscle mass differs substantially from what is typical for their age and sex. Keep that in mind; it lies behind many eGFR misunderstandings in athletes.
Which equation does your lab use? One example is the 2021 CKD-EPI equation, which was developed without race.[5] Its creatinine version was developed from 10 studies with 8,254 participants and its creatinine–cystatin C version from 13 studies with 5,352 participants, and both were checked against measured GFR in a separate validation set of 12 studies with 4,050 participants.[5] In that validation, the new equations combining creatinine and cystatin C were more accurate than the new creatinine-only equations.[5] KDIGO now advises against using race in eGFR calculations and recommends that the same equation be used consistently within a geographical region.[4]
Why eGFR Still Matters: Medication Dosing
Despite its limitations, eGFR is a genuinely useful measure when medication decisions have to be made. On its own, the serum creatinine value is generally less informative for this than an estimate of kidney function calculated with an equation, for example when deciding whether a drug dose needs to be reduced because of impaired kidney function. Because eGFR takes the patient’s age and sex into account, it gives a more practical basis for dosing and dose reductions, at least in Finland in my experience. In my clinical work, the kidney-related effects of medications are mainly assessed using a GFR-based estimate rather than creatinine alone, although some dosing methods use other equations that also contain creatinine.
This is in line with the KDIGO guideline, which considers validated creatinine-based eGFR equations appropriate for drug dosing for most people and clinical settings. Where more accuracy is needed, for example when eGFRcr may be unreliable, it notes that equations combining creatinine and cystatin C, or measured GFR, may be indicated, and that eGFR not indexed for body-surface area may be indicated in people with extremes of body weight.[4]
Some kidney function estimates used in drug dosing also take the patient’s body weight into account. An equation that does not need weight is nevertheless convenient in practice, because a patient’s age and sex are almost always known, so the estimate is easy to calculate. In my experience, eGFR without weight is usually accurate enough for everyday dosing decisions, except in muscular, athletic and large-framed people, and in critical situations a more precise approach may be needed. My impression is that the patients I see with impaired kidney function are more often older and in poorer physical condition than athletic or particularly muscular. Low muscle mass in older, frail patients can, however, itself make creatinine-based estimates less reliable, and kidney disease can also occur in muscular and physically active people, so body composition still has to be considered individually.
If a person’s size differs markedly from the average, however, it is worth taking into account, for example in an exceptionally large person. At the other extreme, an exceptionally frail, small and slight patient may have low creatinine, which can in turn produce an unusually high eGFR. A very frail older woman, for example, may weigh about the same as a child. In that case, body size may be relevant for drug dosing, but taking weight or body size into account does not on its own correct the problem, because the low creatinine caused by low muscle mass can still make the creatinine-based estimate unreliable.
What Is a Normal eGFR? The GFR Categories Explained
KDIGO groups an eGFR blood test result into six categories:[2]
| Category | eGFR (mL/min/1.73 m²) | KDIGO description |
|---|---|---|
| G1 | ≥90 | Normal or high |
| G2 | 60–89 | Mildly decreased |
| G3a | 45–59 | Mildly to moderately decreased |
| G3b | 30–44 | Moderately to severely decreased |
| G4 | 15–29 | Severely decreased |
| G5 | <15 | Kidney failure |
Two details in the guideline matter more than the table itself. First, “mildly decreased” in G2 is defined relative to the young adult level, and without evidence of kidney damage neither G1 nor G2 fulfils the criteria for CKD.[2] Second, CKD is defined as an abnormality of kidney structure or function that has been present for at least three months and has implications for health. That means either a GFR below 60 (categories G3a–G5) or a marker of kidney damage, such as a urine albumin-to-creatinine ratio (ACR) of 30 mg/g or more.[2]
Does age change what is normal? KDIGO acknowledges that the threshold of 60 sits well below the average GFR of young adults, and that GFR declines on average with age, but with substantial variation between individuals, so not everyone loses filtration capacity as they get older.[2] The guideline does not support age-adjusted definitions of CKD.[4]
Is a very high eGFR better? Not necessarily. In the large population data summarised in the KDIGO guideline, creatinine-based eGFR showed a J-shaped relationship with several outcomes, with risk increasing at values above 105 mL/min/1.73 m². The guideline explains that very low creatinine often reflects poor health, such as frailty or sarcopenia, which limits creatinine production.[2] For the same reason, creatinine-based eGFR overestimates GFR in older adults with sarcopenia.[2] A flattering number can therefore hide low muscle mass rather than signal excellent kidneys.
When patients ask me about a flagged result, I usually explain that eGFR is calculated from creatinine, and that the equation does not take into account factors such as body weight, muscle mass, an athletic build or recent exercise. As a result, eGFR can fall outside the reference range even when creatinine on its own is within it. In my clinical experience, this is more common in men, and many of the patients concerned are either large-framed or muscular.
What Does a Low eGFR Mean? Why One Result Is Not a Diagnosis
A single low eGFR on a blood test tells you that something deserves a second look, not that you have kidney disease. KDIGO advises repeating the tests when a low eGFR is found incidentally, in order to confirm whether CKD is actually present.[4] It also warns against assuming a problem is chronic on the basis of one abnormal eGFR, because the finding could reflect a recent acute kidney injury or acute kidney disease.[4]
Check urine albumin too. KDIGO recommends testing people at risk with both urine albumin and an assessment of GFR.[4] Albuminuria is graded as A1 (ACR below 30 mg/g), A2 (30–300 mg/g) and A3 (above 300 mg/g),[2] and an ACR of 30 mg/g or more on a random sample should be confirmed with a first-morning midstream sample.[4] For athletes, timing matters here too, because urine albumin can rise temporarily after strenuous exercise: in a study of 22 marathon runners, urine albumin peaked in the sample taken immediately (within 30 minutes) after the race and was significantly higher than 24 hours before and 24 hours after it.[6] I cover this in detail in my article on proteinuria after exercise.
Why routine screening matters. CKD is often silent, and people may have no symptoms at early stages.[4] KDIGO supports early detection in people at high risk, including those with hypertension, diabetes and cardiovascular disease.[2]
How much change is meaningful? For people already diagnosed with CKD, KDIGO states that a change in eGFR of more than 20% on a subsequent test exceeds the expected variability and warrants evaluation.[4] The practical lesson for everyone is that eGFR moves around from test to test, and trends are often more informative than any single value.
When eGFR is flagged, the first questions I ask are why it is outside the reference range and by how much. If it sits only just beyond the reference limit, and previous results, urine albumin, risk factors and the clinical picture do not point to a problem, follow-up with repeat testing is often a reasonable first step. It is also worth looking at creatinine itself: if creatinine is only slightly above its upper limit, it is reasonable to consider whether the sample was taken fasting or whether something else is genuinely going on in the background. A small deviation should not be dismissed simply because it is small, but in practice, at this stage we usually monitor the patient and repeat the tests.
In my practice, the discussion usually turns to impaired kidney function when creatinine rises, and eGFR falls, clearly beyond the reference range. Kidney disease can, however, be present even when eGFR is relatively preserved if there are other markers of kidney damage, such as albumin in the urine, which is one reason I often order a urine albumin-to-creatinine ratio before that point. When kidney values are abnormal, the possible cause also needs to be considered; KDIGO advises establishing the cause of CKD using the clinical context, personal and family history, laboratory measures and other findings.[4] Diabetes is one of the causes I look for. In a person with diabetes, kidney disease is assessed from the clinical context, eGFR and the urine albumin-to-creatinine ratio. In my practice, a urine glucose check can add information in selected cases, and some of these patients also have glucose and clearly increased protein in their urine, but urine glucose alone does not establish diabetic kidney disease.
If kidney values remain abnormal on repeated testing, the next steps in my experience depend on how abnormal they are, how much albumin is in the urine, how quickly the values are changing and what the suspected cause is. KDIGO lists specific circumstances in which adults with CKD should be referred to specialist kidney care,[4] and a nephrology referral is made when the findings warrant specialist assessment, not simply because a mildly abnormal value stays outside the reference range. Before the nephrologist’s assessment or consultation, further tests are usually done in primary care. These may include cystatin C, a urine albumin-to-creatinine ratio and blood electrolytes, and sometimes a 24-hour urine albumin collection, with the tests chosen according to the clinical findings and the suspected cause.
Why eGFR Can Look Wrong: Factors Outside the Kidneys
Because creatinine-based eGFR depends on creatinine, anything that changes creatinine without changing filtration can distort the eGFR on your blood test. KDIGO explicitly states that interpreting serum creatinine requires consideration of dietary intake.[4]
A meat meal before the test. In one study, eating cooked meat raised median serum creatinine (kinetic Jaffe method) from 80.5 µmol/L before the meal to 101.0 µmol/L one to two hours afterwards, while meat-free meals had little effect and cystatin C was generally unaffected by food.[7] The authors concluded that CKD classification should be based on samples taken fasting or after avoiding cooked meat on the day of sampling.[7]
I also see a related pattern in my own practice: fasting creatinine often sits close to the upper limit of the laboratory reference range. My interpretation is that a patient who has not eaten or drunk as usual before a fasting sample may be mildly dehydrated, which may push creatinine up slightly. The meat study above did not examine this, so it is a clinical impression rather than a research finding. Because eGFR is calculated from creatinine together with age and sex, even a modest change in creatinine can move eGFR across its reference limit, so a borderline eGFR may be flagged even when creatinine is close to or within its own reference interval, and patients often ask me about it. In my experience, many of these borderline results turn out to reflect non-kidney factors such as muscle mass, regular exercise, large body size or hydration rather than kidney disease, but that conclusion should be reached through appropriate assessment, not assumed.
Body composition, diet, supplements and drugs. KDIGO lists situations where creatinine-based eGFR becomes less accurate because of non-kidney influences on creatinine. These include eating disorders, above-knee amputation, spinal cord injury, extreme sport, exercise or bodybuilding, low-protein, keto, vegetarian and high-protein diets, creatine supplements, anabolic or hormonal steroids, and medications that reduce tubular secretion of creatinine.[4]
The laboratory method. The way creatinine is measured can matter too: the KDIGO guideline lists reported examples of substances that may cause analytical interference in creatinine assays.[2]
Cystatin C as a second opinion. Cystatin C is another filtration marker, but it is affected by a different set of factors. KDIGO notes that creatinine may mislead at extremes of body habitus, whereas cystatin C is influenced by factors such as steroid use, thyroid disease and cancer; because neither marker is perfect, combining them gives more accurate estimates when compared with measured GFR.[2] The guideline therefore recommends the combined creatinine–cystatin C eGFR when creatinine-based eGFR is less accurate and the GFR result affects clinical decisions.[4] It also points out that the direction and size of the difference between the creatinine-based and cystatin C–based estimates can itself be informative.[4] For a deeper look at this marker, see cystatin C in athletes.
Cystatin C is a useful test, but in my experience it is not used as a first-line kidney test. That role still belongs to creatinine, in my view probably for historical and cost reasons, even though creatinine comes with a risk of misinterpretation. In the laboratories I work with, cystatin C is not included as standard in many panels and usually has to be ordered separately, although some kidney-focused laboratory panels include it automatically. Availability and routine use vary between laboratories and health systems.
What eGFR Means for Athletes
If you train seriously, the general rules for reading eGFR in a blood test still apply, but your creatinine starts from a different place.
More muscle, more creatinine. In a study of 170 healthy adults (92 women and 78 men), those with moderate or intense physical activity had higher fat-free (lean) mass (55.3 vs 48.5 kg in the study’s body-composition table) and higher serum creatinine (1.04 vs 0.95 mg/dl) than sedentary individuals, along with higher protein and meat intake.[8] Lean mass was significantly related to serum and urinary creatinine but not to cystatin C, even after adjusting for protein and meat intake and physical activity. The authors concluded that cystatin C may be a more appropriate way to assess kidney function in people with higher muscle mass when mild impairment is suspected.[8] KDIGO makes the same point at guideline level, listing extreme sport, exercise and bodybuilding among the situations where creatinine-based eGFR is less accurate and noting that cystatin C–based eGFR may be appropriate when increased muscle mass is the only abnormality.[4] I go through the creatinine side of this in creatinine in athletes.
General reference ranges fit athletes poorly. A review in Sports Medicine concluded that serum creatinine in athletes is related to body mass index, so general-population reference ranges should not be recommended in sports medicine, and that predicting GFR in athletes with creatinine-based equations is questionable.[9] The same review reported an individuality index for creatinine of about 0.33, roughly half of the 0.60 lower limit generally considered necessary for a population-based reference interval to be useful. The authors therefore suggested monitoring athletes with consecutive creatinine measurements and comparing in-season values with a baseline recorded during the recovery period.[9] In plain terms, your own baseline can be more informative than a general-population reference range.
Creatine supplements. A 2025 systematic review and meta-analysis included 21 studies. In its serum creatinine analysis (177 participants taking creatine and 263 controls), creatine supplementation was associated with a small but statistically significant increase in serum creatinine, whereas the pooled GFR analysis of 5 studies (69 creatine and 74 control participants) showed no statistically significant difference.[10] The authors advised interpreting raised creatinine in supplement users with caution and considering more specific markers of kidney function.[10]
In my clinical work, I also see a distinct group of patients who take creatine to increase their muscles’ creatine phosphate stores. Many of them report better strength-training results, and some notice slight bloating. Their blood creatinine, however, may stay elevated for a long time, and in that situation creatinine-based eGFR may not give a reliable picture of kidney function. In practice, a mildly raised creatinine in this setting is often not interpreted as impaired kidney function on its own, provided the rest of the assessment fits. If an accurate answer is needed, cystatin C is usually the next test I would consider.
Recent hard exercise. The same Sports Medicine review lists transient exercise-induced changes in creatinine among the factors that must be accounted for when interpreting athletes’ results.[9] In the same study of 22 marathon runners, serum creatinine also peaked in the sample taken immediately after the race and was significantly higher than both 24 hours before and 24 hours after the race; 82% of the runners had a creatinine rise equivalent to stage 1–2 acute kidney injury by AKIN criteria.[6] That study is also a useful reminder not to dismiss every post-race rise as artefact: the runners showed urine sediment findings of tubular injury and rises in injury and repair biomarkers, which the authors interpreted as suggesting structural damage to the renal tubules after the marathon.[6] Formula choice, exercise timing and exercise-associated acute kidney injury are covered in depth in my article on eGFR in athletes.
When an athlete’s kidneys genuinely need attention. Being fit does not exempt anyone from kidney disease, and the same KDIGO criteria apply: a persistently low eGFR, raised urine albumin or other markers of kidney damage need proper evaluation. If CKD is confirmed, KDIGO recommends moderate-intensity physical activity for a cumulative total of at least 150 minutes per week, or to a level compatible with cardiovascular and physical tolerance.[4] Two other points are directly relevant to athletes: the guideline advises avoiding high protein intake (above 1.3 g/kg body weight per day) in adults with CKD at risk of progression, and reviewing and limiting over-the-counter medicines and dietary or herbal remedies that may be harmful in CKD.[4]
Conclusion: What Is eGFR in Blood Test, and What Should You Do With Yours?
eGFR is a calculated estimate of kidney filtration, usually derived from serum creatinine, and it is a useful screening number on a routine blood test. Chronic kidney disease is common, with an estimated 697.5 million cases of all-stage CKD worldwide in 2017,[1] and it is often silent in its early stages.[4]
At the same time, eGFR is only as good as the creatinine behind it. Muscle mass, a meat meal and creatine supplements can push creatinine up and eGFR down without necessarily reflecting any change in kidney filtration, and recent hard exercise can raise creatinine temporarily too, although a post-race rise is not always harmless. The answer is neither to ignore a low result nor to panic over it. Depending on the clinical context, appropriate assessment may include repeat testing under standardized conditions, a urine albumin test, comparison with previous results or your own baseline, and cystatin C when creatinine-based eGFR may be unreliable.
In my clinical work, eGFR is flagged almost daily, and in my experience many of these flags, after appropriate assessment, turn out to reflect non-kidney factors affecting creatinine rather than kidney disease. When values are clearly or persistently abnormal, the work-up usually moves on to a urine albumin-to-creatinine ratio if it has not already been done, cystatin C, an assessment of possible causes such as diabetes and, when the findings warrant it, a nephrology referral. Despite its limitations, eGFR also remains the practical yardstick for adjusting medication doses to kidney function for most people, as long as body size and muscle mass are considered in unusually large or very frail patients.
For athletes in particular, the most useful question is not simply whether the value is inside the reference range, but whether the result makes sense for this body, this training load and this testing situation.
References
[1] https://pubmed.ncbi.nlm.nih.gov/32061315/
[2] https://doi.org/10.1016/j.kint.2023.10.018
[3] https://doi.org/10.1056/NEJMra054415
[4] https://doi.org/10.1016/j.kint.2023.10.016
[5] https://doi.org/10.1056/NEJMoa2102953
[6] https://pmc.ncbi.nlm.nih.gov/articles/PMC5526736/
[7] https://pubmed.ncbi.nlm.nih.gov/17270090/
[8] https://pubmed.ncbi.nlm.nih.gov/18235143/
